Armature Uneven deforming with Modifiers?
One can't parent a mesh to multiple armatures, and 1 should realy be enough.
If there are some small morphing you'd like to do who are because of their detail movement not easily done with bones, you can add shapekeys for that.

Notice its also possible to use bone constrains (often with empties as helper objects) to control you bones, ea i did that in a car rig, just telling because i dont see any reason why 2 armatures would be needed. I made way more complex armatures but have never had a need for it, so it puzzles me why you want it.
Its a but unclear to me, what you try to achieve with it, perhaps if you can explain i might think of another way to do that.